This invention relates to an aqueous coating composition having improved adhesion to friable surfaces such as chalky weathered paint surfaces and masonry surfaces. More particularly, this invention relates to an aqueous coating composition including an emulsion polymer of selected composition having a glass transition temperature (Tg) of xe2x88x9220 C to 100 C and an average particle diameter less than 120 nanometers, and 0.25-10 wt. % of a water-soluble alkoxylated amine. And the invention relates to a method for improving the adhesion of a dried aqueous coating composition to a friable surface by forming an aqueous coating composition including an emulsion polymer of selected composition having a glass transition temperature (Tg) of xe2x88x9220 C to 100 C and an average particle diameter less than 120 nanometers, and 0.25-10% of a water-soluble alkoxylated amine; applying the aqueous coating composition to a friable surface; and drying, or allowing to dry, the aqueous coating composition.
The present invention serves to provide a dried coating which has improved adhesion to a friable surface. Coatings are frequently desirably applied to surfaces which are both porous and weak, i.e., subject to attrition on abrasion such as, for example, chalky surfaces of coatings which have weathered to an extent that poorly consolidated pigment forms a surface layer on the coating and masonry surfaces, weathered or not, which have a poorly consolidated surface. A substrate to which a coating is applied may have an entirely friable surface or only portions of the surface may be friable. Such substrates present a problem to the applicator in that, without being bound by this mechanism, the aqueous coating composition may not penetrate the weak boundary layer of the friable surface or friable surface areas sufficiently to provide a dry coating with the requisite degree of adhesion to the substrate below the weak surface.
U.S. Pat. No. 4,771,100 discloses the use of ethoxylated fatty amines in the preparation of latexes containing about 0.1 to 10 weight percent of copolymerized carboxylic acid monomer which have particle sizes between 889 and 1091 Angstroms for use as coatings. Improved adhesion to friable surfaces is desired.
Adhesion to a substrate to which it has been applied is a generally desirable characteristic of a coating. However, some surfaces are notoriously difficult to adhere to and coatings which adhere well to sound surfaces will fail to adhere to such surfaces. One such difficult surface is a friable surface, that is, one on which a weak, poorly bound, inadequately consolidated surface layer such as a badly chalking weathered paint surface or a brittle, crumbling masonry surface, is to be coated. The problem faced by the inventors is the provision of a suitable aqueous coating composition and a method for improving the adhesion of a coating so that that adhesion to friable surfaces can be effected. We have now found that that certain polymer compositions used in conjunction with water-soluble alkoxylated amines provide improved adhesion to friable surfaces relative to alternative compositions.
In a first aspect of the present invention there is provided an aqueous coating composition having improved adhesion to friable surfaces including an emulsion polymer having a glass transition temperature of xe2x88x9220 C to 100 C and an average particle diameter less than 120 nanometers, the emulsion polymer having at least one copolymerized ethylenically unsaturated nonionic monomer, each of the nonionic monomer(s) having a water solubility less than 8%, and at least one copolymerized acid monomer, such that the acid number of the emulsion polymer is 30 to 100; and 0.25-10%, by weight based on the emulsion polymer weight, water-soluble alkoxylated amine.
In a second aspect of the present invention there is provided an aqueous coating composition having improved adhesion to friable surfaces including an emulsion polymer having a glass transition temperature of xe2x88x9220 C to 100 C and an average particle diameter less than 120 nanometers, the emulsion polymer having 8-99.5%, by weight based on the weight of the emulsion polymer, of at least one copolymerized ethylenically unsaturated first nonionic monomer, each of the first nonionic monomer(s) having a water solubility of 8% or more, 0-91.5%, by weight based on the weight of the emulsion polymer, of at least one copolymerized ethylenically unsaturated second nonionic monomer, each of the second nonionic monomer(s) having a water solubility of less than 8%, and at least one copolymerized acid monomer, such that the acid number of the emulsion polymer is 4 to 100; and 0.25-10%, by weight based on the emulsion polymer weight, water-soluble alkoxylated amine.
In a third aspect of the present invention there is provided a method for improving the adhesion of a dried aqueous coating composition to a friable surface including forming an aqueous coating composition including an emulsion polymer having a glass transition temperature of xe2x88x9220 C to 100 C and an average particle diameter less than 120 nanometers, the emulsion polymer having at least one copolymerized ethylenically unsaturated nonionic monomer, each of said nonionic monomer(s) having a water solubility less than 8%, and at least one copolymerized acid monomer, such that the acid number of the emulsion polymer is 30 to 100, and 0.25-10%, by weight based on polymer weight, water-soluble alkoxylated amine; applying the aqueous coating composition to a surface; and drying, or allowing to dry, the aqueous coating composition.
In a fourth aspect of the present invention there is provided a method for improving the adhesion of a dried aqueous coating composition to a friable surface by forming an aqueous coating composition including an emulsion polymer having a glass transition temperature of xe2x88x9220 C to 100 C and an average particle diameter less than 120 nanometers, the emulsion polymer having 8-99.5%, by weight based on the weight of the emulsion polymer, of at least one copolymerized ethylenically unsaturated first nonionic monomer, each of the first nonionic monomer(s) having a water solubility of 8% or more, 0-91.5%, by weight based on the weight of the emulsion polymer, of at least one copolymerized ethylenically unsaturated second nonionic monomer, each of the second nonionic monomer(s) having a water solubility of less than 8%, and at least one copolymerized acid monomer, such that the acid number of the emulsion polymer is 4 to 100, and 0.25-10%, by weight based on polymer weight, water-soluble alkoxylated amine; applying the aqueous coating composition to a surface; and drying, or allowing to dry, the aqueous coating composition
The aqueous coating composition contains a waterborne emulsion polymer. The emulsion polymer contains at least one copolymerized nonionic ethylenically-unsaturated monomer, such as, for example, a (meth)acrylic ester monomer including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, aminoalkyl (meth)acrylate; styrene or substituted styrenes; butadiene; vinyl acetate or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, N-vinyl pyrollidone; (meth)acrylonitrile and (meth)acrylamide. The use of the term xe2x80x9c(meth)xe2x80x9d followed by another term such as acrylate or acrylamide, as used throughout the disclosure, refers to both acrylates and acrylamides and methacrylates and methacrylamides, respectively.
The water solubility of the nonionic monomers incorporated into the emulsion polymers herein are defined as those determined using the Quantitative Structural Activity Relationship (QSAR) program. The program uses the molecular structure to estimate physical-chemical properties including, molecular weight, vapor pressure, solubility, bioconcentration factor, hydrolysis half-life, Henry""s coefficient, partitioning data, and other parameters( based on Lyman, W., Reehl, W., and Rosenblatt, D. Handbook of Chemical Property Estimation Methods. Chapter 2 xe2x80x9cSolubility in Waterxe2x80x9d. McGraw Hill Book Co., New York, 1982). The QSAR database used to calculate the water solubility assessment is maintained by the Institute for Process Analysis, Montana State University (Bozeman, Mont., USA) and accessed through Tymnet Data Systems and Numerica Online Systems (Numericom. 1994. The Online Interface for Numerica Users. Technical Data Base Services, Inc. (TDS, 135 West 50th Street, New York, N.Y. 10020). Some water solubilities are presented in Table 1.
The emulsion polymer has a certain acid number range resulting from at least one copolymerized monoethylenically-unsaturated acid monomer such as, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, sulfoethyl methacrylate, phosphoroethyl methacrylate, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, and maleic anhydride. The acid number of the emulsion polymer of the first and third aspects of the present invention is 30 to 100, preferably 30 to 50, more preferably 39 to 50. The acid number of the emulsion polymer of the second and fourth aspects of the present invention is 4 to 100, preferably 8 to 50.
The emulsion polymer used in this invention is substantially thermoplastic, or substantially uncrosslinked, when it is applied to the surface, although low levels of deliberate or adventitious crosslinking may be present. When low levels of precrosslinking or gel content are desired low levels of nonionic multi-ethylenically unsaturated monomers such as, for example, 0.1%-5%, by weight based on the weight of the emulsion-polymerized polymer, allyl methacrylate, diallyl phthalate, 1,3-butylene glycol dimethacrylate, 1,6-hexanedioldiacrylate, and divinyl benzene may be used. It is important, however, that the quality of the film formation is not materially impaired.
The polymerization techniques used to prepare emulsion polymers are well known in the art. In the preparation of emulsion polymers conventional surfactants may be used such as, for example, anionic and/or nonionic emulsifiers such as alkali or ammonium alkyl sulfates, alkyl sulfonic acids, fatty acids, and oxyethylated alkyl phenols. The amount of surfactant used is usually up to 6% by weight, based on the weight of total monomer. Either thermal or redox initiation processes may be used. Conventional free radical initiators may be used such as, for example, hydrogen peroxide, t-butyl hydroperoxide, and ammonium and/or alkali persulfates, typically at a level of 0.05% to 3.0% by weight, based on the weight of total monomer. Redox systems using the same initiators coupled with a suitable reductant such as, for example, sodium bisulfite may be used at similar levels. Chain transfer agents such as, for example, alkyl mercaptans may be used in order to moderate the molecular weight of the polymer.
In another aspect of the present invention the emulsion polymer may be prepared by a multistage emulsion polymerization process, in which at least two stages differing in composition are polymerized in sequential fashion. Such a process usually results in the formation of at least two mutually incompatible polymer compositions, thereby resulting in the formation of at least two phases within the polymer particles. Such particles are composed of two or more phases of various geometries such as, for example, core/shell or core/sheath particles, core/shell particles with shell phases incompletely encapsulating the core, core/shell particles with a multiplicity of cores, and interpenetrating network particles. In all of these cases the majority of the surface area of the particle will be occupied by at least one outer phase and the interior of the particle will be occupied by at least one inner phase. Each of the stages of the multi-staged emulsion polymer may contain the same monomers, surfactants, chain transfer agents, etc. as disclosed herein-above for the emulsion polymer. In the case of a multi-staged polymer particle the Tg for the purpose of this invention is to be calculated by the Fox equation as detailed herein using the overall composition of the emulsion polymer without regard for the number of stages or phases therein. Similarly, compositional quantities for a multi-staged polymer particle such as, for example, the amount of first nonionic monomer and the acid number shall be determined from the overall composition of the emulsion polymer without regard for the number of stages or phases therein. The polymerization techniques used to prepare such multistage emulsion polymers are well known in the art such as, for example, U.S. Pat. Nos. 4,325,856; 4,654,397; and 4,814,373.
The emulsion polymer has an average particle diameter less than 120 nanometers, preferably less than 100 nanometers, more preferably less than 80 nanometers, most preferably less than 70 nanometers. Particle sizes herein are those determined using a Brookhaven Model BI-90 particle sizer manufactured by Brookhaven Instruments Corporation, Holtsville N.Y. Reported as xe2x80x9ceffective diameterxe2x80x9d.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the emulsion polymer is xe2x88x9220xc2x0 C. to 100xc2x0 C. Tgs used herein are those calculated by using the Fox equation (T. G. Fox, Bull. Am. Physics Soc., Volume 1, Issue No. 3, page 123(1956)). that is, for calculating the Tg of a copolymer of monomers M1 and M2,
1/Tg(calc.)=w(M1)/Tg(M1)+w(M2)/Tg(M2)
wherein
Tg(calc.) is the glass transition temperature calculated for the copolymer
w(M1) is the weight fraction of monomer M1 in the copolymer
w(M2) is the weight fraction of monomer M2 in the copolymer
Tg(M1) is the glass transition temperature of the homopolymer of M1
Tg(M2) is the glass transition temperature of the homopolymer of M2,
all temperatures being in xc2x0K.
The glass transition temperatures of homopolymers may be found, for example, in xe2x80x9cPolymer Handbookxe2x80x9d, edited by J. Brandrup and E. H. Immergut, Interscience Publishers.
The aqueous coating composition contains 0.25-10 wt. %, preferably 0.5-8 wt. %, more preferably 1-8 wt. %, of a water-soluble alkoxylated amine, by which is meant herein an amine substituted with one, two, or three xe2x80x94(RO)xRxe2x80x2 groups, where R is C1-C4 alkyl or mixtures thereof, mixtures disposed randomly or in sequences (blocks), preferably ethyl, and where x is from 5-100. Further, the amine may be substituted with 0-2 Rxe2x80x3 groups, where Rxe2x80x3 is a C1-C24 alkyl, aralkyl, or aromatic group, preferably each Rxe2x80x3 group is a C1-C24 alkyl selected such that the Iodine number of the water-soluble alkoxylated amine is less than 30, more preferably such that the Iodine number of the water-soluble alkoxylated amine is less than 15, inorder to minimize the color of the alkoxylated amine. Preferred are t-amines. In any event the alkoxylated amine is water-soluble at least to the amount that it is utilized in the aqueous coating composition at 25 C. Typical alkoxylated amines are the commercially available alkoxylated t-amines, Ethox SAM-50, Ethomeen 18/25, and the primary alkoxylated amine, Jeffamine M-2070.
The amount of pigment in the aqueous coating composition may vary from a pigment volume concentration (PVC) of 0 to 75 and thereby encompass coatings otherwise described, for example, as clear coatings, semi-gloss or gloss coatings, flat coatings, and primers.
The aqueous coating composition is prepared by techniques which are well known in the coatings art. First, if the coating composition is to be pigmented, at least one pigment is well dispersed in an aqueous medium under high shear such as is afforded by a COWLES(copyright) mixer or, in the alternative, at least one predispersed pigment may be used. Then the emulsion polymer, selected surfactant and alkyl polyglycoside is added under low shear stirring along with other coatings adjuvants as desired. Alternatively, either or both of the selected surfactant and alkyl polyglycoside may have been previously added to the emulsion polymer before, during, or subsequent to the preparation of the emulsion polymer. Alternatively, the emulsion polymer may be present during the pigment dispersion step. The aqueous coating composition may contain conventional coatings adjuvants such as, for example, emulsifiers, buffers, neutralizers, coalescents, thickeners or rheology modifiers, freeze-thaw additives, wet-edge aids, humectants, wetting agents, biocides, antifoaming agents, colorants, waxes, and anti-oxidants. The aqueous coating composition may contain up to 75%, by weight based on the total dry weight of the polymer, of an emulsion polymer not meeting the limitations of the emulsion polymer of the first or second aspect of the present invention.
The solids content of the aqueous coating composition may be from 25% to 60% by volume. The viscosity of the aqueous polymeric composition may be from 50 KU (Krebs Units) to 120 KU as measured using a Brookfield Digital viscometer KU-1; the viscosities appropriate for different application methods vary considerably.
The presence and amount of friable material on a surface can be determined using the method of ASTM test method D-659. In this test method the lower the rating the more friable material present. The dry coating compositions of this invention have been evaluated and are beneficially used over substrates having surfaces with a rating of 3 or less. A xe2x80x9cfriable surfacexe2x80x9d herein is defined as one which has a rating of 3 or less determined by the above method. An alternative approach to determining the presence and amount, actually the depth, of friable material, is to repeatedly adhere a piece of tape onto an area of the surface and remove the friable material. This is continued until no more friable material is visually detected on the tape. At that point the depth can be determined quantitatively by using a suitable microscopic technique such as scanning electron microscopy. Using this test method we found that the test substrates of the examples had at least 10 microns of friable material on their surfaces.
Conventional coatings application methods such as, for example, brushing, rolling, and spraying methods such as, for example, air-atomized spray, air-assisted spray, airless spray, high volume low pressure spray, and air-assisted airless spray may be used in the method of this invention. The aqueous coating composition may be advantageouly applied to substrates such as, for example, weathered paint and friable cementitious substrates such as, for example, stucco and mortar but may also be applied to other architectural substrates. Drying is typically allowed to proceed under ambient conditions such as, for example, at 0xc2x0 C. to 35xc2x0 C.